1. Field of the Invention
This invention relates to measuring air flow.
2. Prior Art
Known airflow meters include a hot film anemometer sensor. In accordance with such hot film wire or hot film anemometry, a thin film wire sensor is deposited on the substrate such as quartz or glass. It is also known to use a very fine platinum or tungsten wire freely supported or wound on a ceramic bobbin and maintained at a certain temperature above the intake air temperature by electronic sensing and feedback circuits. Any change in the airflow alters the cooling effect of the air on the heated wire. An electronic circuit can sense this change in heat transfer rate and record changes in the heating current to maintain the temperature of the wire at a set value.
Hot film anemometer sensors have generally had a low speed of response as a result of the low thermal conductivity of the quartz glass or fiber. Further, great care must be taken in handling the fine wire or quartz fiber in manufacturing the sensor elements. This results in limited manufacturing production capacity and increased unit costs.
U.S. Pat. No. 4,129,848 issued to Frank et al teaches a platinum film resistor device. A layer of quartz, deposited upon an insulative substrate, is sputter etched to produce etch pits in the surface thereof. A layer of platinum is deposited over the quartz layer. A second layer of quartz is deposited over the layer of platinum and the second layer of quartz is masked and chemically etched away in the regions where the platinum layer is to be removed. The exposed platinum and a portion of the second quartz layer are then sputter etched away leaving the platinum in a predetermined configuration.
A typical mass airflow system for an automotive application uses one thermal sensor element and cannot distinguish the direction of airflow. Under certain operating conditions, however, such as high acceleration rates and heavy loads at low RPM when there is intake and exhaust valve overlap in certain engines, there is a net flow of air in the reverse direction for a brief period of time. Since the fuel metering for a cycle is dependent upon the air charge of the previous one, a backflow condition can result in significant errors if this contribution is not separated from the normal mass airflow signal. Attempts have been made to infer the amount of backflow by analysis of the electronic signal from the airflow systems, but this has not been totally satisfactory.
There still remains a need for an airflow sensor for detecting backflow and having a good thermal conductivity which can be made at a very low unit cost with high reliability. Further, it would be advantageous to have such a backflow air sensor which reduces system costs and improves overall system reliability as well as increasing packaging efficiency. These are some of the problems this invention overcomes.